The present invention relates to photomasks employed for producing integrated circuits of high integration density, e.g., large-scale integrated circuits (LSI), very large-scale integrated circuits (VLSI), etc., and to photomask blanks used to produce such photomasks. More particularly, the present invention relates to a phase shift photomask and a photomask blank therefor.
Semiconductor integrated circuits, e.g., IC, LSI, VLSI, etc., are produced by repeating a lithography process in which a resist is coated on a substrate to be processed, e.g., a silicon wafer, and this resist is subjected to exposure by a stepper or other exposure system to form a desired pattern thereon, followed by development, etching, doping, CVD, and so forth.
With the achievement of high performance and high integration density of semiconductor integrated circuits, photomasks used in such a lithography process have increasingly been demanded to have a high degree of accuracy. For example, in the case of DRAMs, which are of a typical type of LSI, a 5.times.reticle for 1M-bit DRAMs, that is, a photomask having a pattern of size which is 5 times the size of a pattern to be formed on a wafer by exposure, is demanded to have a dimensional accuracy of 0.15 .mu.m even for a mean value of .+-.3.sigma. (.sigma.: standard deviation). Similarly, 5.times.reticles for 4M-bit DRAMs, 16M-bit DRAMs and 64M-bit DRAMs are demanded to have dimensional accuracies of from 0.1 .mu.m to 0.15 .mu.m, from 0.05 .mu.m to 0.1 .mu.m, and from 0.03 .mu.m to 0.07 .mu.m, respectively.
In addition, there is an increasing demand for reduction in the line widths of device patterns formed by using these reticles: 1.2 .mu.m for 1M-bit DRAMs; 0.8 .mu.m for 4M-bit DRAMs; 0.6 .mu.m for 16M-bit DRAMs; and 0.35 .mu.m for 64M-bit DRAMs. To comply with these demands, various exposure methods have heretofore been studied.
However, device patterns for future advanced devices, e.g., 64M-bit DRAMs, cannot be realized by the conventional stepper exposure method that employs a photomask because of the resist pattern resolution limit of this method. To overpass this limit, a reticle based on the new idea of phase shift photomask has been proposed, as disclosed, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 58-173744 and Japanese Patent Application Post-Exam Publication No. 62-59296. Phase lithography that employs a phase shift photomask is a technique whereby the resolution and contrast of the projected image are improved by controlling the phase of light passing through a reticle.
In the phase shift lithography, high-precision control of the phase shift angle is demanded in order to improve the resolution and contract of the projected image during the pattern transfer. The phase shift angle is determined by the film thickness and refractive index of the phase shifter pattern. In particular, the control of the film thickness is a serious problem in production of a phase shift photomask. Etching of the phase shifter layer is performed by the dry etching method, which is excellent in fine processing of high accuracy. However, the method whereby the film thickness of the phase shifter pattern can be controlled most reliably is to select materials for the phase shifter layer and the underlying layer so that the ratio (selectivity) of the dry etch rate of the phase shifter layer to that of the underlying layer is satisfactorily high, and to carry out dry etching for a time longer by a specific time than a length of time considered necessary for the phase shifter layer to be completely removed, that is, to perform overetching. With this method, a uniform etch depth is obtained with high accuracy over the entire surface of the substrate. Overetching may also be necessary for arranging the pattern cross-section in the desired configuration.
However, when quartz and commercially available coating glass (Accuglass 211S, manufactured by Allied Signal) are employed as a glass substrate and a phase shifter material, respectively, the dry etch selectivity is undesirably low, i.e., only about 1.5 to 2.0 at the highest, so that even if minimum required overetching is performed, the glass substrate is etched to such an extent that the phase shift angle is affected.
For this reason, it is indispensably necessary in order to control the phase shift angle with high accuracy to provide on the glass substrate a so-called etching stopper layer made of a material having a sufficiently high etch selectivity relative to the phase shifter layer. As a substance which can function as such an etching stopper layer, a transparent electrically conductive film is known which is mainly composed of tin oxide, as shown, for example, in Japanese Patent Application Post-Exam Publication No. 61-61664.
However, the transparent electrically conductive film, which is mainly composed of tin oxide, absorbs i-rays from a mercury-arc lamp used as illuminating light for pattern transfer. Therefore, an intensity of illuminating light adequate for practical use cannot be obtained at regions where the light-shielding layer is selectively removed unless the film thickness is held down to a level of about 15 nm.
However, the dry etching resistance of the above-described transparent electrically conductive film is not sufficiently high. Therefore, the film of about 15 nm in thickness is completely removed by overetching performed for about 20 minutes, resulting in a failure to function as an etching stopper layer.
Accordingly, a phase shift photomask produced from such a photomask blank makes it impossible to perform sufficient overetching for the phase shifter layer. Therefore, it becomes difficult to effect precise phase control.
In addition, since the film which absorbs light as described above decreases in thickness during the overetching process, as described above, an in-plane transmittance distribution undesirably occurs between regions where the phase shifter film has been selectively removed by the overetching and regions where the phase shifter layer remains unremoved.